The present invention relates to articles of footwear, and in particular to articles of footwear having a sole with improved cushioning characteristics.
One of the primary focuses in the recent design of athletic footwear has been underfoot cushioning. This is primarily because, while the human foot has existing natural cushioning characteristics, such natural characteristics are alone incapable of effectively overcoming the stresses encountered during everyday activity. For example, an athlete may partake in an activity in which substantial loads are placed on the foot, joint, and muscular structures of the leg including the ankle, knee, and hip joints. Such activities include road running, track running, hiking or trail running. Trail running in particular can subject the foot and lower extremities to extreme conditions and therefore extreme loads. As one example, in trail running, as distinguished from track and road running, one might encounter rough terrain such as rocks, fallen trees, gravel or steep hills. Traversing this terrain necessarily involves large stresses to be borne by the foot. Even in less demanding environments, such as in ordinary walking or road running, the human foot still experiences significant stresses. Cushioning systems have therefore developed to mitigate and overcome these stresses.
Existing cushioning systems for footwear have tended to focus on mitigating vertical ground reaction forces in order to offset the impact associated with heel strike during gait. This is not altogether unreasonable, considering that, in some activities, the body experiences peak forces nearing 2000 N in the vertical direction. Yet, during running, walking, trail running or the like, a heel strike typically involves both vertical and horizontal forces. In fact, due to the angle of the foot and leg upon contact with the ground, up to thirty (30) percent of the forces generated are in the horizontal plane.
Many traditional cushioning systems also suffer from the problem of preloading, due in part to the nature of such cushioning systems' design. Specifically, a significant amount of existing cushioning systems utilize a continuous midsole in which each section of the midsole is susceptible to compression upon contact with the ground. In other words, traditional midsoles are continuous such that, when one portion of the midsole is compressed, an adjacent portion is also compressed. This results in large areas of the midsole being compressed at the time of ground contact, thus reducing cushioning potential and forcing the midsole to act as a monolithic structure.
Yet another concern with existing cushioning systems is that, while different cushioning systems must satisfy similar objectives, such systems often need to be tailored to a particular activity or use being undertaken. For example, the demands and needs of a trail runner in terms of cushioning may be vastly different than the demands of a casual walker. The trail runner, for instance, may have specific needs that require more substantial cushioning than the ordinary walker. In fact, in trail running protection from bruising, which may be caused by repeated impacts with rocks, roots and other irregularities, is a major concern. Quite differently, during walking and/or road running, a premium is placed on vertical compression and a stable platform.